I used to have a 3ph post hole digger that didn't do much good, as I didn't have the downpressure required to dig in my tough soil.

I've priced renting a bobcat for a weekend to drill my postholes for my current project. I know it will do the job, but I end up doing some fencing each year.

If I fabricated a rig to put a 2 man gas powered post hole auger on my quick-tach on the fel, how much could I jack myself up?

I was going to fabricate a 'utility' quick-tach plate with a hitch receiver, mounts for my light snowplow, and some other attachment points. If I welded up bracketry and a throttle extension for one of those 5-8hp auger units, and gently used my fel to move it into place and apply slight down pressure/control, do you guys think it would work? Obviously, with a FEL I could easily destroy the auger, but if I was careful?? We have a hard clay and the downward pressure is really needed.

I commercially auger holes--up to 36" and 13' deep--in clay and through frost. I use a skidsteer though.

Sounds to me like you are not using the right auger. I mean it should have either carbide or at the least forged teeth. My extreme-duty augers are rated to go through asphalt and they only have forged teeth. I'd go back and use the 3pt PHD and install a down pressure cylinder, or add some weight to it directly over the head. OR, install the 3pt PHD from its mount and fab a bracket/mount off the one side of the FEL bucket. Power the PHD via a hydraulic motor (or use a gear pump which is a motor) Keep in mind you will have uneven pressure on one side of the FEL so you'll need to compensate for it by strapping a weight inside the bucket and opposite the auger head as a counterweight---otherwise you could flip yourself over sideways either pushing or pulling on the auger.
Those two-man augers are not designed to take more than what two men can handle either laterall or vertical loading bearingwise---I broke a crankshaft on my 5hp 2-man after it hit some stones.

And the other thing to consider with augers is the flighting thickness--even though mine are extreme duty I have managed to bend mine and they're about 3/8" thick--2-man auger flights are measured in gauge--as in sheetmetal. And the drive tube of th auger may only be 3/4"-1" in diameter WITH a shear pin. Bigger augers may be 2" hex drive or 3" round drive (some smaller/larger). My hydraulic auger drive head is rated at 1,700 lb.ft. torque.

Another option--I have a 2hp 12VDC auger head--supposedly a "one-man" (yeah--one man and TWO gorillas). Has jumper cable-style leads. This sucker is STRONG. I chain or ratchet strap it to another pole/post/truck in case it gets away from me. It is direct-drive--no clutch. It has an 8" auger but it has a 3/8" shear bolt on a 3/4" drive. They are expensive--about $1,000 for a complete setup inc. tax and shipping--but it's portable and quiet and not real heavy. My-D Hand-D makes it. Link below

I did better when I kept the biting edge of the flights sharp, had a helper keep watering the clay, and turned slowly. In the worst area I'd start with a small auger, then hand-dig a few inches so the wider auger would start true, then use it to go to depth. If you decide on using the loader, I imagine it's steadier than two guys standing on the ground.